There are commonly known a wide variety of conventional plows which are capable of operating underwater for trenching and pipe laying operations on a seabed. As used herein, the term seabed shall include the bottom of any body of water including for example, a lake, river or ocean. Typically, such plows include a share or some other type of soil cutting device for forming a trench and are dragged behind a tow vehicle, such as a ship or submarine tractor, by means of a suitable cable. For a variety of reasons, including avoidance of undersea obstacles, the path required to be plowed is often circuitous, including numerous changes in direction.
There are two basic methods by which the plow direction can be varied. The simplest approach, particularly in the case of plows which are dragged behind a tow vehicle, is to vary the direction of the tow vehicle so that the towing direction exerted on the plow by its cable causes the plow to follow a new path. However, this crude "tow and follow" method of controlling the plow direction has several disadvantages including a certain amount of difficulty in maintaining precise control over the path of the plow. This can present a particular problem where it is necessary to position a pipeline or cable with some accuracy along a designated course. As an improvement to the basic tow and follow system described above, systems have been devised which are comprised essentially of a plow towed by a long cable, split at the lowered end into a bridle whose tow ends are then attached to the plow. By differentially pulling on the two ends of the bridle, a turning moment is produced which slightly angles the plow share from its previous track, and causes a change in the plowing direction. By using a mechanism which always causes the centerline of the tow cable to pass through the plow's "center of resistance" (usually at the location of the soil cutting member), the differential force required to act on the bridle can be minimized. Various systems for achieving this result are disclosed in U.S. Pat. No. 4,759,138 to Grinsted. While such bridle control type systems are an improvement over more basic systems, they suffer from several problems. Most significantly, plows which establish an effective towing point located at the plow's center of resistance suffer a significant degree of straight line tracking instability, since the towing point tends to be located rearward relative to the front of the plow. This tracking instability is an undesirable side-effect of these types of steering system which must be designed to translate the relatively small steering forces imparted by the tow cable, to the plow. Such straight line tracking instability is an undesirable characteristic as it tends to cause the plow to veer off course under conditions where a straight trenching path is desired. Further, such plows tend to be rather complicated and are limited in the degree of precision which can be achieved due to the fact that the turning moment caused by the bridle control will be effected by the uncertain position of the long tow cable.
Others have attempted a more direct approach to steer an underwater plow. For example U.S. Pat. No. 4,329,793 to Reece discloses a plow having angular adjustable skids which have soil engaging blades. However, that system also has a serious disadvantage to the extent that the turning moment generated by the relatively small soil engaging blades may be insufficient to adequately control the plow direction, particularly where the soil conditions are soft and the trench to be cut is relatively deep. Moreover, the design disclosed in Reece also suffers from the straight line tracking instability problem described above, since the effective tow point is located toward the rear of the plow, above the center of resistance.
Thus, it is an object of the invention to provide a steerable underwater plow having exceptional straight line tracking capability. Further, it is another object of the invention to provide a plow having excellent maneuverability to accommodate trenching operations where frequent and substantial changes in plowing direction are required.